In April of 1941, laborers began building the concrete behemoths in La Rochelle's harbor. The massive pens were to hold the "Unterseeboots," one of Germany's most formidable weapons during World War II.
Because of their essential place in Nazi warfare, the U-boat pens had to be built in secret. Most of the construction was performed at night, illuminated by spotlight that would be extinguished at the hint of danger, leaving the workers (mostly Spanish and Portuguese prisoners of war) in pitch darkness. By 1942 the pens could hold 13 U-boats. With a complicated series of locks, underwater pens could be drained, and the submarines hauled up by crane to workshops.
Due to its advantageous position on the Atlantic, some of the most serious U-boat missions of WWII were launched from La Rochelle pens. The city remained under German control until 1945, one of the last French cities to be liberated by the Allies, after which the U-boat pens sat empty. Today, they are intermittently used by the French navy, and as such are not open to the public. They are, however, visible from the adjacent road and hard to miss.